1. Field of the Invention
The present invention relates to a blow molding, in particular to the one provided with an ornamental strip on its outer surface, and its molding process.
2. Description of the Related Arts
Conventionally automobiles and ships are provided with long moldings on their side surfaces for protective and ornamental purposes. Automobiles are also provided with belt-like bumpers on their front and rear part for shock adsorbing function. Those molded products, moldings or bumpers as described above will be hereinafter referred to as a long molded product.
Some of those long molded products, generally produced through injection molding or extrusion molding, may be integrally bonded with an ornamental strip such as stainless or color resin tie plates on their outer surfaces for improving ornamental effects. Furthermore, long molded products with a hollow interior, formed by a blow molding process have been produced in increasing quanities because of their light weight and excellent shock absorbing function as disclosed in Japanese Patent Application Laid-Open No. 206241/1984, and Japanese Patent Publication No. 22123/1988.
The above molding is obtained through the blow molding process by integrally bonding the ornamental strip and the blow molded body, both of which are formed of totally different materials. After the blow molding, or while using the molding, warpage often occurs in its longitudinal direction, or a gap is formed between the blow molded body and the ornamental strip owing to different expansion ratio thereof.
The molding obtained through the blow molding process is hollow shaped, and generally formed of a synthetic resin. At blow molding, the tubular parison extruded from a die head will cause the molecular configuration to be aligned in the extrusion direction, the so called orientation phenomena. The molding, then largely expands in the longitudinal extruded direction of the parison. That is why the cavity of the die for blow molding is designed to have a longer length than the desired length of the molding, considering the amount of molding shrinkage. For example, in order to obtain the molding with its length of 1 m, the cavity of the die is designed to have its length of 1.014 m, which incorporates the molding shrinkage amount of 1.4%.
While using the molded product, since the synthetic resin used for the molded product and the material for the ornamental strip have different coefficients of linear expansion, they will independently expand to the change of outside temperature. At blow molding, the blow molded body is likely to shrink in its longitudinal direction due to molding shrinkage. The ornamental strip integrally bonded with the blow molded body, however, will prevent such shrinking, resulting in causing warpage in the longitudinal direction. While using the molding, a gap may be formed between the blow molded body and the ornamental strip because of the different coefficients of linear expansion, which adversely affects the appearance of the molding and further causes separation of the ornamental strip from the blow molded body.